1. Field of the Invention
The present invention relates to lasers, and particularly to frequency-converted solid-state lasers and cooling systems for such lasers.
2. Description of Related Art
A typical solid-state laser includes a laser cavity formed by two opposing mirrors, a solid state gain medium situated within the laser cavity, and an optical pump source for pumping the gain medium. One important type of solid-state laser is a diode-pumped solid-state laser, which typically includes a rare earth doped solid-state gain medium pumped by optical radiation from a laser diode. A variety of important applications have motivated research into diode-pumped solid-state lasers, including laser display systems, optical data storage, medical instrumentation, and communications. For small to moderate levels of optical power, one particularly useful diode-pumped solid-state laser is a microlaser, which comprises a short element (i.e. less than about five mm) of a solid-state gain medium positioned in an optically resonant laser cavity. The pump beam is supplied by a semiconductor diode laser, which pumps the solid-state gain medium to provide energy to support laser operation within the laser cavity. The optically resonant laser cavity is defined by two opposing reflective surfaces, which may be formed directly on opposing ends of the solid-state gain medium. The resulting structure is compact and reliable.
Relative to other lasers, optically-pumped solid-state lasers can have many advantages including greater efficiency, smaller size, and lower cost. However, it would be advantageous to improve the efficiency of solid state lasers even further in order to reduce energy consumption, reduce waste heat, and ensure a consistent, reliable output. Efficiency improvements could provide a higher power laser at a lower cost, with obvious commercial advantages.
Many problems stand in the way of achieving higher efficiency; for example, a high temperature within any of the optical elements can adversely affect performance, and if the temperature gets too high, the laser may lose its ability to lase, and destruction of the optical element may result. Additionally, heating can optically distort the laser emission due to thermal and stress-induced birefringence. Temperature is an inherent problem in high power optically pumped lasers, because heat is unavoidably produced within the gain medium during absorption of optical pumping radiation. Also, to a much lesser extent, heat is produced by absorption of lasant radiation. Therefore, high power lasers generally produce more waste heat, which must be dissipated by a heat removal system. Problems associated with high temperature, and previous cooling systems are discussed for example, in Ser. No. 08/784,051, filed Jan. 17, 1997, now U.S. Pat. No. 5,796,766, which is hereby incorporated by reference herein.
Many solid state lasers include a nonlinear crystal that converts a fundamental laser emission to a second frequency. Such frequency-converted lasers are important because they allow access to wavelengths that would otherwise be unavailable; for example the 1064 nm near-infrared line of Nd:YAG can be frequency-doubled to the green at 532 nm. In many frequency-converted lasers, it is useful to maintain the nonlinear crystal within a predetermined temperature range, in order to optimize the efficiency of frequency conversion.
One problem with frequency-converted lasers can be caused by thermal gradients within the nonlinear crystal. Such thermal gradients can reduce efficiency in the frequency-converted output. Background information regarding the frequency conversion process is discussed, for example, in Ser. No. 08/760,702, filed Dec. 5, 1996 entitled "Efficient Frequency Converted Laser" by Hargis and Nelte, now U.S. Pat. No. 5,761,227 issued Jun. 2, 1998, which is hereby incorporated by reference herein.